The NASA moon rocket stands ready less than 24 hours before it is scheduled to launch at the Kennedy Space Center in Cape Canaveral, Florida, on Sunday. AP
The 32-story-tall, two-stage Space Launch System (SLS) rocket and its Orion crew capsule were due for blast-off from the Kennedy Space Center in Cape Canaveral, Florida, during a two-hour launch window opening at 8:33am EDT (1233 GMT).
The maiden voyage of the SLS-Orion, a mission dubbed Artemis I, is intended to put the 5.75-million-pound vehicle through its paces in a rigorous demonstration flight, pushing its design limits, before NASA deems it reliable enough to carry astronauts.
Billed as the most powerful, complex rocket in the world, the SLS represents the biggest new vertical launch system the US space agency has built since the Saturn V flown during the Apollo moon program of the 1960s and '70s.
The Service Module component of the Orion spacecraft for the Artemis 2 mission is shown at Kennedy Space Center. AFP
The spacecraft was slowly trundled to historic Launch Pad 39B earlier this month following weeks of final preparations and ground tests. Last week, NASA officials concluded their flight readiness review declaring all systems were "go for launch."
One issue cited by NASA officials last week as a potential show stopper for Monday's launch would be any sign during rocket fueling that a newly repaired hydrogen line fitting had failed to hold.
If the countdown clock is halted for any reason, NASA has set Sept. 2 and Sept. 5 as backup launch dates.
Barring last-minute technical difficulties or unfavorable weather, Monday's countdown should end with the rocket's four main R-25 engines and its twin solid-rocket boosters igniting to produce 8.8 million pounds of thrust, about 15% more thrust than produced by the Saturn V, sending the spacecraft streaking skyward.
About 90 minutes after launch, the rocket's upper stage will thrust Orion out of Earth orbit on course for a 42-day flight that brings it to within 60 miles of the lunar surface before sailing 40,000 miles (64,374 km) beyond the moon and back to Earth. The capsule is expected to splash down in the Pacific on Oct. 10.
A screen displays various live views of the Artemis I unmanned lunar rocket at the Kennedy Space Center, Florida. AFP
Although no humans will be aboard, Orion will be carrying a simulated crew of three - one male and two female mannequins - fitted with sensors to measure radiation levels and other stresses that real-life astronauts would experience.
A top objective for the mission is to test the durability of Orion's heat shield during re-entry as it hits Earth's atmosphere at 24,500 miles (39,429 km) per hour, or 32 times the speed of sound, on its return from lunar orbit - much faster than more common re-entries of astronaut capsules returning from low-Earth orbit.
"That's our highest priority that we have to accomplish," lead flight director Rick LaBrode said of demonstrating the heat shield's ability to withstand re-entry friction, expected to raise temperatures outside the capsule to nearly 5,000 degrees Fahrenheit (2,760 Celsius). "That's what's going to keep the capsule together and save the astronauts."